1. Field of the Invention
The present invention generally relates to mixer circuits and communication apparatuses employing a mixer circuit, and more particularly to a direct demodulation circuit and a communication apparatus employing a direct demodulation circuit.
2. Description of the Related Art
Regarding a communication system having a low-frequency component in a baseband of a broadband such as WCDMA (Wideband Code Division Multiplex Access), it has recently become desirable to use a direct demodulation circuit without any external filter part so as to realize miniaturization, low cost and low electric power consumption with respect to a receiver. A direct demodulation circuit has the problem that a weak local current mixed into an RF signal is detected together with the RF signal and there arises a DC offset.
It is possible to reasonably construct a small subharmonic pumping mixer employing an APDP (anti-parallel diode pair) in which diodes are connected in anti-parallel. Also, when the frequency of the local current is set as half of a carrier frequency, it is possible to perform direct demodulation. At the same time, even if the local current with half of the carrier frequency leaks an RF signal, it is impossible for the local current to return and become a DC component. As mentioned above, the subharmonic pumping mixer has the advantage that the DC offset is not generated in principal. Thus, the subharmonic mixer can be used as a direct demodulation mixer.
FIG. 1 shows a structure of a subharmonic pumping mixer.
An RF is supplied to an APDP 10 formed with diodes D1 and D2 via a filter formed with an L1 and a C1. In a down-conversion mixer, a local Lo is additionally supplied to the APDP via a filter formed with an L2 and a C2. When the APDP demodulates the RF, the demodulated signal is supplied to a terminal of a resistor R via a low-pass filter formed with an L3 and a C3.
FIG. 2 shows a characteristic of the subharmonic pumping mixer in FIG. 1 with respect to a conversion gain. As is shown in FIG. 2, the conversion gain has a maximum of no more than about −20 DB within a range in which local power is increased.
FIG. 3 shows a characteristic of the subharmonic pumping mixer in FIG. 1 with respect to a noise index. As is shown in FIG. 3, the noise index has a minimum of no less than about 20 DB.
As mentioned above, while a conventional subharmonic pumping mixer has the advantage that it is possible to implement the subharmonic pumping mixer with low cost and no DC offset arising due to a leak of a local signal, the subharmonic pumping mixer has some problems with respect to the gain and the noise characteristics.
Therefore, it is necessary to develop a small size, low cost and a low electric power consuming mixer circuit capable of suppressing any DC offset caused by a local leak signal and achieving better gain and noise characteristics.